A constant warning time device (often referred to as a crossing predictor or a grade crossing predictor in the U.S., or a level crossing predictor in the U.K.) is an electronic device that is connected to the rails of a railroad track and is configured to detect the presence of an approaching train and determine its speed and distance from a crossing (i.e., a location at which the tracks cross a road, sidewalk or other surface used by moving objects). The constant warning time device will use this information to generate a constant warning time signal for controlling a crossing warning device. A crossing warning device is a device that warns of the approach of a train at a crossing, examples of which include crossing gate arms (e.g., the familiar black and white striped wooden arms often found at highway grade crossings to warn motorists of an approaching train), crossing lights (such as the red flashing lights often found at highway grade crossings in conjunction with the crossing gate arms discussed above), and/or crossing bells or other audio alarm devices. Constant warning time devices are often (but not always) configured to activate the crossing warning device at a fixed time (e.g., 30 seconds) prior to an approaching train arriving at a crossing.
Typical constant warning time devices include a transmitter that transmits a signal over a circuit formed by the track's rails and one or more termination shunts positioned at desired approach distances from the transmitter, a receiver that detects one or more resulting signal characteristics, and a logic circuit such as a microprocessor or hardwired logic that detects the presence of a train and determines its speed and distance from the crossing. The approach distance depends on the maximum allowable speed of a train, the desired warning time, and a safety factor. Preferred embodiments of constant warning time devices generate and transmit a constant current AC signal on said track circuit; the constant warning time devices detect a train and determines its distance and speed by measuring impedance changes caused by the train's wheels and axles acting as a shunt across the rails, which effectively shortens the length (and hence lowers the impedance) of the rails in the circuit. Multiple constant warning devices can monitor a given track circuit if each device measures track impedance at a different frequency. Measurement frequencies are chosen such that they have a low probability of interfering with each other while also avoiding power line frequencies (e.g. 60 Hz) and traction power frequencies (e.g., 25 Hz, 100 Hz) as well as second and third harmonics thereof.
As is known in the art, railroad tracks are installed using railroad ties and a trackbed consisting of stone or other suitable material referred to as a ballast. The ballast is packed between, below, and around the ties and is used to allow water drainage, bear the load from the railroad ties, and to hold the track in place as trains roll by. The track length and ballast determine the upper frequency that a constant warning time device can use to reliably determine train position. It is known that the ballast can degrade and/or cause current leakage over time. Moreover, the ballast condition can change due to the weather conditions. Therefore, the ballast can have a resistance that effects the impedance of the train circuit, making the prediction of an approaching train more difficult and prone to errors. Thus, due to the shunting action of the ballast, train position is more accurately detected at low frequencies.
The track circuit of interest for the constant warning time device is the “approach” (i.e., the area from a termination shunt to the crossing), which is confined in frequency by the termination shunts. The lower the frequency the larger the shunt. Practically speaking, however, termination shunts have a lower frequency limit due to their physical size. For example, shunts become prohibitively large, which is undesirable, when trying to get down to a low frequency of e.g., 45 Hz. Even if the track circuit could accommodate the larger shunts, they are still not desirable because of the costs and manpower required to replace existing shunts. As such, larger termination shunts should not be used to overcome the effects caused by the ballast conditions.
Thus, there is a need and desire for a reliable and accurate train prediction determination that can account for, and overcome the problems associated with, changing ballast conditions.